SEQUENTIAL TUNNELING IN DOPED SUPERLATTICES - FINGERPRINTS OF IMPURITY BANDS AND PHOTON-ASSISTED TUNNELING

We report a combined theoretical and experimental study of electrical transport in weakly coupled doped superlattices. Our calculations exhi bit negative differential conductivity at sufficiently high electric f ields for all dopings. In low-doped samples the presence of impurity b ands modifies the current-voltage characteristics substantially, and w e find two different current peaks whose relative height changes with the electron temperature. These findings can explain the observation o f different peaks in the current-voltage characteristics with and with out external THz irradiation in low-doped samples. From our microscopi c transport model we obtain quantitative agreement with the experiment al current-voltage characteristics without using any fitting parameter s. Both our experimental data and our theory show that absolute negati ve conductance persists over a wide range of frequencies of the free-e lectron laser source.